Spotlight on Optics

Highlighted Articles from OSA Journals

September 2011

Spotlight Summary by Brynmor Davis

Complete characterization of supercontinuum coherence

The femtosecond or attosecond timescales exhibited in ultrafast pulses pose a challenge for robust pulse characterization, as the response time of optical detectors is significantly longer than the required measurement resolution. Despite this difficulty, methods for the measurement of deterministic ultrafast pulses are now relatively mature and the major controversies in the field are (mostly) resolved. Comprehensive measurement of the properties of random ultrafast fields remains a largely open problem however. In this article, Genty et al. consider the probabilistic description of one of the most important and interesting nondeterministic ultrafast fields—the supercontinuum light that can be generated using an ultrafast source and nonlinear fibers.

Experimental characterization of supercontinuum fields has revealed both fine ultrafast structure and significant pulse-to-pulse randomness, the latter indicating a need for measurement of optical coherence (i.e., statistical) properties. Standard means of measuring coherence do not provide an unambiguous statistical characterization of supercontinuum fields, as the typical simplifying assumption of stationarity (loosely interpreted as a constancy of the statistical properties with time) cannot be applied. While statistical measurements specific to nonstationary fields have been taken, to date they have also provided a highly incomplete field characterization. By contrast, nonstationary coherence theory provides a number of concepts for the thorough characterization of supercontinuum fields but does not give practical means for experimental measurement of these descriptors.

In this paper Genty et al. provide an important step toward bridging the gap between general nonstationary coherence theory and experimental characterization of supercontinuum fields. Using a model of supercontinuum generation, the authors calculate expected second-order coherence functions and propose a decomposition of the field into two components—a "quasi-stationary" and a "quasi-coherent" contribution. Additionally, they discuss how this decomposition may be leveraged in experimental measurements. The next step for this work is clearly an experimental investigation of the theoretical and numerical results presented. If the predictions presented in this paper can be validated, it will represent a significant advance in the understanding of the nondeterministic aspects of supercontinuum generation.